The invention relates to a diagnostic circuit for measuring the resistance and the leakage current of at least one electrical load, specifically a firing cap of a motor vehicle restraint system. With such electrical loads in the form of firing caps, there is frequently a requirement to measure the resistance and the leakage current for diagnostic purposes in order to ensure correct functional capability of the firing caps. This diagnosis can be carried out at regular or irregular intervals, for example when the motor vehicle is started. If it is determined during the diagnosis that the firing cap resistance lies outside the permissible tolerance range and/or an unacceptable leakage current occurs, a fault message can be promptly output and/or the driving parameters (driver voltage, driver current, driving time) for driving the firing cap can be adapted if required.
During the diagnostic measurement, the firing cap resistance and the firing cap leakage current are usually measured one after the other. However, if a multiplicity of firing caps are provided, which is frequently the case in modern motor vehicle restraint systems, a long period of time is required to test all of the firing caps.
German Patent DE 196 38 393 C1 discloses a diagnostic circuit in which the firing cap that is to be tested is supplied by a power source which is driven in broadband fashion. The voltage drop across the firing cap is applied, using a switching element, to a correlator that is also supplied in broadband fashion. In order to measure leaks, further separate power sources may be provided for measuring leakage currents to ground or battery voltage.
U.S. Pat. No. 5,640,095 describes a circuit for testing the leakage resistance of a switching point, in which a test current is fed into the switching point and the voltage occurring at the switching point is detected. In order to perform a bipolar leakage resistance measurement, two measuring currents in opposite directions can be fed into the switching point.
Published European Patent Application EP-A 0 486 114 discloses an apparatus for testing an electrical circuit with respect to the leakage current and the continuity. The circuit is part of a bridge circuit whose branches are supplied with identical currents. To detect the leakage current, the bridge voltage is evaluated.
It is accordingly an object of the invention to provide a diagnostic circuit for measuring the resistance and the leakage current of at least one firing cap which overcomes the above-mentioned disadvantageous of the prior art apparatus of this general type. In particular, it is an object to provide a diagnostic circuit that enables a precise diagnosis to be carried out quickly.
With the foregoing and other objects in view there is provided, in accordance with the invention, a diagnostic circuit for measuring resistance and leakage current for at least one firing cap of a motor vehicle occupant protection system. The diagnostic circuit includes: a terminal for receiving a reference potential; at least one load having two terminals, one of the two terminals of the at least one load connected to the terminal for receiving a reference potential; an activation circuit for feeding a current to the at least one load during a diagnostic measurement; a resistance measuring circuit for determining a resistance of the at least one load; a leakage current measuring circuit for determining a leakage current that may be flowing during the diagnostic measurement, the leakage current measuring circuit configured to measure a difference between the current fed to the at least one load by the activation circuit and a current flowing from the at least one load to the terminal for receiving a reference potential; and a plurality of electrical power supplies connected to the leakage current measuring circuit and to the two terminals of the at least one load.
In the diagnostic circuit, the electrical resistance of the at least one load, in particular of the firing cap, and the leakage current are measured by detection circuits which are preferably at least partially embodied separately. These measurements can be carried out simultaneously. The time interval which is required overall for measuring the resistance and leakage current is thus extremely short. In particular, if a multiplicity of loads are to be tested, for example, if more than 10 firing cap circuits, are present, the overall diagnostic time interval required can be shortened drastically. During the simultaneous determination of leakage currents, the resistance measurement result based on a current measurement can additionally be corrected immediately taking into account the leakage current, with the result that the measuring accuracy is increased.
The leakage current measurement can be determined easily by forming a difference between the current supplied on the input side and the current measured on the output side. This differential current is preferably detected by a resistive voltage divider which generates a voltage representing the leakage current at its tap. Here, two electrical power supplies which are connected to one another and which simulate the current supplied to the firing cap or flowing out of the firing cap are provided. A connection point between these electrical power supplies can be connected to the leakage current measuring circuit, preferably to the resistance voltage divider. The resistive voltage divider ensures that the measuring current occurring at the connection point lies between the reference voltages applied to the resistive voltage divider and increases or decreases depending on the direction of the leakage current. Instead of a resistive voltage divider, it is, however, also possible to provide a single resistor which is connected to ground, for example. In this case, the leakage current measuring circuit changes it""s polarity depending on the direction of the leakage current.
In accordance with an added feature of the invention, a voltage regulator that regulates the voltage occurring at a load terminal to a specific value is connected to one of the electrical power supplies. In this way, the current and voltage conditions at the load are permanently predefined and the electrical power supply connected to this terminal is controlled, in contrast to the electrical power supply connected to the other load terminal, by an impressed voltage and not by an impressed current.
In accordance with an additional feature of the invention, the voltage is variable, with the result that, for example, it is possible to repeat leakage current measurements with different potential values. In this way, leakage resistances to potential values between ground and the supply voltage are reliably detected.
In accordance with an another feature of the invention, the diagnostic circuit is used in vehicle occupant protection systems for motor vehicles, in particular airbag systems. The diagnostic circuit can be integrated directly into the control unit of the restraint system. This permits rapid diagnosis with short transient recovery times.
In accordance with a further feature of the invention, an electrical power supply (current mirror circuit) is provided which exactly simulates or which provides a fixed proportion of the current flowing through the firing cap during the diagnosis. The current is preferably impressed by a power source, and conducts it to the resistance measuring circuit and leakage current measuring circuit via separate terminals. This permits current measurement without affecting and falsifying the current which is actually flowing through the firing cap. Reciprocally, undesired effects occurring during the measurement of the resistance and leakage current can also be prevented by the electrical power supply terminals.
In accordance with a further added feature of the invention, the currents supplied to the resistance measuring circuit and the leakage current measuring circuit correspond to just a fraction, for example a tenth, of the current supplied to the firing cap. In this way, on the one hand, it can be ensured that the firing cap is supplied with current sufficient for reliable diagnosis, and on the other hand, that only small currents have to be coped with during the evaluation. These low currents additionally ensure that the power drain is low and that there is no excessive thermal loading of the evaluation circuit. In addition, only low currents have to be built up and switched. In particular, when a multiplexer which is preferably of analog design is used, and there is a time-selective measurement of a multiplicity of firing caps, the power consumption is greatly reduced.
In accordance with a further additional feature of the invention, the resistance of the firing cap is measured by separately measuring the current flowing through the firing cap. The current is preferably simulated by the electrical power supply, and by voltage drop occurring across the firing cap during this process.
In accordance with yet an added feature of the invention, different currents are generated for the resistance and leakage current measurements. The leakage current is preferably measured with only the lower current strength, because undesired power losses become particularly clearly apparent then.
In accordance with yet an additional feature of the invention, there is preferably one multiplexer for switching between different loads. Consequently, in each case, just one of the resistance and leakage current detection components has to be provided, which not only reduces the expenditure on circuitry, but also ensures identical measuring conditions, sensitivity, measuring tolerances etc. and permits the system to be extended as desired. Standardization, too, only has to be carried out once.
In accordance with yet a further feature of the invention, precise resistance measurement with leakage current compensation and/or offset compensation can be carried out by duplicating the measurement with different current strengths and forming the difference between the measured results.
In accordance with yet another feature of the invention, a multiplicity of loads, in particular firing caps, can be measured using the same resistance and leakage current measuring circuit components. The multiplexer is configured in particular as an analog multiplexer and has switches which are connected in series with the individual firing caps so that individual testing of the firing caps can be carried out successively. The voltage drops across the firing caps are preferably measured by a voltage detector, for example an operational amplifier. This operational amplifier is also preferably connected via switches of the multiplexer to the terminals of all of the loads which are to be tested. This gives rise to a somewhat higher expenditure for switches than would be the case if the operational amplifier were directly connected to the electrical power supply terminals of the multiplexer switches that are connected in series with the loads. Another possibility would then be implemented without additional switches. However, with this circuitry it is then possible to measure the load voltage directly in a precise way without any errors as a result of voltage drops at the multiplexer switches connected in series to the loads. Preferably, additional diodes are provided between the operational amplifier and the loads, through which diodes the undesired effects due to back currents or the like are prevented.
In accordance with yet another added feature of the invention, the voltage regulator, if one is present, is connected directly to a terminal of the voltage detector. The voltage regulator is consequently connected, via the multiplexer switch which has just closed, to one of the terminals of the firing cap which has just been measured, with the result that the potential occurring there is regulated.
In accordance with yet another additional feature, the invention can be used for all electrical loads in which the resistance (impedance) and the leakage current are to be measured. It is suitable, and is designed, in particular for firing cap diagnosis of a multiplicity of firing circuits for firing caps. In that case, it is possible to perform very rapid diagnostic measurement with short transient recovery times while at the same time providing the possibility of measuring leakage currents and firing cap resistances. It is possible to use a multiplexer which is of simple design and can be controlled in an analog fashion. The requirements placed on the analog multiplexers in terms of circuitry and control are very low because only a small number of switches are required per firing cap (four switches per firing cap). The other circuit components can be used jointly to carry out all of the diagnostic measurements and therefore, only one of them has to be provided. The invention can also be embodied in the form of an integrated circuit and in this case requires only a small silicon area.
With the foregoing and other objects in view there is provided, in accordance with the invention, a motor vehicle occupant protection system that includes: a terminal for receiving a reference potential; at least one load having two terminals, one of the two terminals of the at least one load connected to the terminal for receiving a reference potential; an activation circuit for feeding a current to the at least one load during a diagnostic measurement; a resistance measuring circuit for determining a resistance of the at least one load; a leakage current measuring circuit for determining a leakage current that may be flowing during the diagnostic measurement, the leakage current measuring circuit configured to measure a difference between the current fed to the at least one load by the activation circuit and a current flowing from the at least one load to the terminal for receiving a reference potential; a plurality of electrical power supplies connected to the leakage current measuring circuit and to the two terminals of the at least one load; a first integrated circuit including a first plurality of switching elements; and a second integrated circuit including a second plurality of switching elements. The at least one load includes a plurality of firing caps. Each one of the plurality of firing caps is connected between a respective one of the first plurality of the switching elements and a respective one of the second plurality of the switching elements. Each one of the plurality of firing caps is connected to fire when the respective one of the first plurality of the switching elements and the respective one of the second plurality of the switching elements are closed.
When the firing transistor pairs are distributed between different chips, the vehicle occupant protection system has a high level of operational reliability.
With the foregoing and other objects in view there is provided, in accordance with the invention, a motor vehicle occupant protection system that includes: a terminal for receiving a reference potential; at least one load having two terminals, one of the two terminals of the at least one load connected to the terminal for receiving a reference potential; an activation circuit for feeding a current to the at least one load during a diagnostic measurement; a resistance measuring circuit for determining a resistance of the at least one load; a leakage current measuring circuit for determining a leakage current that may be flowing during the diagnostic measurement, the leakage current measuring circuit configured to measure a difference between the current fed to the at least one load by the activation circuit and a current flowing from the at least one load to the terminal for receiving a reference potential; a plurality of electrical power supplies connected to the leakage current measuring circuit and to the two terminals of the at least one load; a first integrated circuit including a first plurality of switching elements; a second integrated circuit including a second plurality of switching elements; a common switching element; and a plurality of switching elements. The at least one load includes a plurality of firing caps. Each one of the plurality of the firing caps has a first terminal and a second terminal. The first terminals of at least some of the plurality of the firing caps are connected together and are driven by the common switching element. Each one of the second terminals of the at least some of the plurality of the switching elements are individually connected to a respective one of the plurality of the switching elements.
The vehicle occupant protection system can fire the caps utilizing a reduced number of firing transistors and leads, but at the same time can provide for the possibility of selective testing.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a diagnostic circuit for measuring the resistance and leakage current of at least one firing cap of a vehicle occupant protection system for motor vehicles, and vehicle occupant protection system of corresponding configuration for motor vehicles, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.